Asymmetric Digital Subscriber Line (ADSL) is a technology which allows for simultaneous voice and data traffic to coexist over a communication channel comprising a standard telephone transmission line. Typically, the standard telephone transmission lines comprise an unshielded twisted pair of copper wire having a gauge of 22-26 AWG. Twisted pairs, which can be used to connect a central telephone system (a `central` unit) to a subscriber's telephone (a `remote` unit) can support bandwidths of up to 2 MHz through the use of digital signal processing (DSP) technology. Thus, they can be used for bandwidth-intensive applications, such as internet access and video-on demand, as well as for carrying voice traffic. Frequency division multiplexing is used so that a plurality of signals, each occupying a different frequency band, can be simultaneously sent over the same transmission line.
The voice traffic band comprises a number of frequency sub-bands, or channels, ranging from DC to 20 KHz. The analog voiceband frequency is typically specified as 200-4000 Hz. Customer specified additions may include phone operation up to 8 KHz and 12-16 KHz billing tones. In addition, DC to 30 Hz frequencies are typically assigned for auxiliary analog signaling purposes, such as ringing the telephone, dial pulsing and on/off hook signaling.
ADSL data traffic bandwidth for CAP (carrierless amplitude and phase) modulation is typically from 35 KHz-1.5 MHz. Of this, upstream data traffic (i.e., remote unit to central unit) uses the 35 KHz-191 KHz band, while the downstream traffic (i.e., central unit to remote unit) uses the 240 KHz-1.5 MHz band. The bandwidth for DMT (discrete multi-tone modulation) starts at 25 KHz.
As both data traffic and voice traffic are sent over the same physical channel, the differing types of signal traffic being received over the transmission line must be distinguished from one another at both the remote unit and the central unit. In addition, voice and data signals to be transmitted over the transmission line must be properly combined at each end. In prior art systems, Plain Old Telephone Service (POTS) separation filters, installed at both the remote unit and at the central unit are used for this purpose.
FIG. 1 shows the typical arrangement of an existing system for handling voice and data traffic over a physical channel comprising a transmission line 100 comprising a twisted pair of wires 104, 106. The existing systems must operate when the phone 118 is either on-hook (POTS signals may include ringing signals and/or on-hook transmissions such as caller-ID) or off-hook (POTS signals may include tone dialing, pulse dialing and voice).
At the remote end 110, the transmission line is connected to a remote's high pass filter (HPF) 112 and a remote's low pass filter (LPF) 114, the filters 112, 114 being arranged in parallel. The output of the remote HPF 112 is then sent to a remote ADSL transceiver 116 which connects to additional data links in a known manner. The output of the remote LPF 114 connects to a telephone 118 or answering machine, or like, in a known manner.
At the central unit 120, the transmission line 100 is connected to a central unit high pass filter (HPF) 122 and a central unit low pass filter (LPF) 124, the filters 122, 124 again being arranged in parallel. The output of the central unit HPF 122 is presented to the central unit ADSL transceiver 126, from which it can connect to additional data links in a known manner. The output of the central unit LPF 124 is presented to a public switched telephone network (PSTN) 128 for connection to other subscribers at other remote systems, long distance services, and the like.
The filters 112, 114, 122 and 124 must meet certain performance criteria. In the ADSL frequency range, the LPF 114, 124 input impedance must be high enough not to load down the transceiver input, which generally has a resistance of 100.OMEGA.. On the other hand, in the voiceband frequency range, the HPFs 112, 122, must have a high enough impedance so as not to load down the telephone 118.
In addition to impedance criteria, the various filters must also meet certain performance specifications. For instance, the LPF filters must meet stopband criteria to prevent POTS signaling from causing errors on the ADSL line. POTS signaling which can create errors include ringing signals (20 Hz), broadband ringing transients caused by central unit relays that apply and remove the ringing signal, on-hook/off-hook transients created by a subscriber picking up a handset to make a call, and a ring trip transient caused by a subscriber at a remote telephone answering an incoming call, among others.
In addition to stopband criteria, the LPF filters must also meet passband (200 Hz-4000 Hz) criteria. These passband criteria include insertion loss (at 1000 Hz), passband ripple, return loss (measure of how close the input impedance matches the off-hook load), envelope delay distortion and longitudinal balance, among others.
In telephone networks which have off-hook termination impedances that are purely real (i.e., no imaginary component) the task of meeting both the passband and stopband performance specifications can be achieved by using only passive filters.
However, in telephone networks where such impedances are complex and not purely real, achieving the passband and stopband performance criteria using only passive filters is very difficult. This is because of the wide variety of potential POTS signals which must be handled by the filtering system. This difficulty is shown in the IEEE article written by John Cook and Phil Sheppard, "ADSL and VADSL Splitter Design and Telephony Performance" (IEEE Journal On Selected Areas in Communications, Volume 13, Number 9, December 1995). In this article, an example passive filter is shown. This was an 8th order modified elliptic low pass filter with a cutoff frequency of 42.5 KHz. The pole was pushed out to 42.5 KHz in order to increase the passband return loss performance to 12 dB at 4 KHz. The point of the exercise was to show that having the pole at 42.5 KHz was unacceptable since this collided with the ADSL data bandwidth. In addition, even with the pole at 42.5 KHz, the 12 dB return loss number was still unacceptable since the specification is 18 dB minimum.
Due to the difficulty of meeting the criteria with only passive filters, active filters are used to make the complex load look real through the use of impedance converters.
U.S. Pat. No. 5,623,543 to Cook is an example of an active filtering approach used to accommodate the various POTS signals. However, such a design adds complexity, cost, power consumption, and board real estate over the traditional passive design.
U.S. Pat. No. 5,627,501 to Biran et al. is an example of a passive filter approach. This design calls for a pair of low pass filters connected in series between the transmission line and the POTS receiver at the remote unit. One of the two low pass filters is always activated, while the second is selectively activated by a control signal created at the remote unit. The control signal detects the attenuation caused by the transmission line, due to the latter's length, and activates (or deactivates) the second filter accordingly. While this design is able to limit current flow through the lowpass filter to prevent saturation of that filter, it requires monitoring of transmission line loss. More significantly, this design does not take into consideration filter performance in both on-hook and off-hook conditions.